1. Field of the Invention
The present invention relates to a vacuum pump, and more particularly to a vacuum pump that can be used in a pressure range from low vacuum to high vacuum and ultra-high vacuum, in an industrial vacuum system that is used in semiconductor manufacturing, high-energy physics and the like.
2. Description of the Related Art
In the present description an example will be explained of a composite-type vacuum pump that is provided with a turbo-molecular pump section and a thread groove pump section. Conventional composite-type vacuum pumps of this type have a structure wherein a turbo-molecular pump section 104 and a cylindrical thread groove pump section 105 are sequentially disposed inside a chassis 103, having an intake port 101 and a discharge port 102, from the intake port 101 side, as illustrated in the vertical cross-sectional diagram of a composite-type vacuum pump in a conventional embodiment illustrated in FIG. 12. FIG. 13 is an enlarged diagram of section B of FIG. 12.
In FIG. 12, the reference numeral 106 denotes a rotating shaft of a rotor 107 of the cylindrical thread groove pump section 105 and the turbo-molecular pump section 104, and the reference numeral 108 denotes a motor that causes the rotating shaft 106 to rotate.
In this conventional composite-type vacuum pump 100, the rotor 107 of the cylindrical thread groove pump section 105 is made of an aluminum alloy. The highest revolutions that the composite-type vacuum pump can achieve are limited thus by the strength of the rotor 107 at the cylindrical thread groove pump section 105.
Such being the case, a cylindrical rotor 109 that results from shaping, to a cylindrical shape, a fiber-reinforced plastic material (fiber-reinforced plastic, ordinarily referred to as “FRP material”), may be used as the rotor in the thread groove pump section of the composite-type vacuum pump. Structures for increasing the strength of such a cylindrical rotor are also known.
As the fiber-reinforced plastic material there can be used, for instance, aramid fibers, boron fibers, carbon fibers, glass fibers, polyethylene fibers and the like.
A combination of dissimilar types of material is thus used in a case where a cylindrical rotor 109 of a fiber-reinforced plastic material (hereafter, “FRP material”) is disposed at the lower end section of the rotor 107 of the turbo-molecular pump section 104 in the composite-type vacuum pump, and hence differences arise in the extent of deformation caused by centrifugal force and by thermal expansion. Therefore, this raised the concern of joint portion loosening, or, contrariwise, the concern of breakage of the cylindrical rotor 109, which is made of an FRP material, on account of a high load acting thereon. In particular, fibers break off at the end face of the cylinder, and hence the strength in the vicinity of the end face is lower than at other portions. This raised the concern of easy breakage of that portion when acted upon by a load.
From the viewpoint of securing concentricity by preventing tilting of the cylindrical rotor 109, and from the viewpoint of weight reduction, the joint portion 110 of the rotor 107 is ordinarily shaped in an L-shaped cross section and comprise a disc-like portion 110a and a joining portion 110b. Such a structure affords a load-relieving effect through deflection of a lower portion side of the joining portion 110b. In an FRP structure, however, there is hardly any deflection in the vicinity of the end face, at which strength is weakest, and hence hardly any load-relieving effect is afforded.
Various conventionally known measures to tackle the above occurrence have been proposed, for instance those disclosed in Japanese Patent No. 3098139 and Japanese Patent Application Publication No. 2004-278512.
In the composite-type vacuum pump of Japanese Patent No. 3098139, specifically, the rotor of the turbo-molecular pump section and the cylindrical rotor of the thread groove pump section are joined to each other by way of a support plate of FRP material in order to mitigate the difference in the extent of deformation caused by centrifugal force and differences in thermal expansion between the turbo-molecular pump section and the thread groove pump section.
In the composite-type vacuum pump disclosed in Japanese Patent Application Publication No. 2004-278512, the winding angle of fibers in the FRP material, as well as shapes and shaping conditions, such as resin content, are so designed as to mitigate the difference in the extent of deformation caused by centrifugal force and differences in thermal expansion between the turbo-molecular pump section and the thread groove pump section.
However, the structure disclosed in Japanese Patent No. 3098139, wherein the rotor of the turbo-molecular pump section and the cylindrical rotor of the thread groove pump section were joined to each other by way of a support plate of an FRP material, was problematic on account of the increased number of parts and greater assembly man-hours that such a structure involved. Moreover, assembly was difficult to achieve with good precision, and the clearance with respect to a fixed section had to be made wider than in a conventional instance, in order to prevent contact with the fixed section. This entailed lower evacuation performance, which was likewise problematic.
In the structure disclosed in Japanese Patent Application Publication No. 2004-278512, i.e. a structure wherein the winding angle of fibers of an FRP material, and shaping shapes and conditions, such as resin content, were variously designed, the shape of the FRP material was a complex one, which was problematic in terms of poorer productivity and higher costs that this entailed.